Many digital services are transmitted in parallel with analog services and this situation is likely to continue for some time. In many cases, the digital services do not necessarily offer duplicate data streams to the corresponding analog services. An example of such an arrangement is where national services may be replicated in those digital and analog services but local programming may only be broadcast on the analog services. It is therefore a common requirement in receivers that means for both digital and analog reception have to be provided.
FIG. 1 of the accompanying drawings illustrates an example of a known type of “dual standard” tuner for receiving terrestrial digital and analog services. An antenna input 1 is connected to a power splitter 2, which is required to buffer the input spectrum to an analog path 3 and a digital path 8 while adding minimum degradation to the signal quality. The analog path 3 comprises an analog tuner 4, which receives and down converts any selected channel to an output intermediate frequency (IF). The output of the analog tuner 4 is connected to a surface acoustic wave filter (SAWF) 5, whose function is to filter out the desired channel and possibly isolate the video and audio information contained within the analog channel. The output of the filter 5 is connected to an analog demodulator 6 for demodulating the selected channel. The output of the demodulator 6 is connected to an output device illustrated as a common display means 7, such as a monitor.
The digital path 8 comprises a digital tuner 9, which down converts the desired channel to another intermediate frequency, which may be the same as or different from the intermediate frequency in the analog path 3. The output of the tuner 9 is connected to a surface acoustic wave filter 10, which filters out the desired channel and provides attenuation to adjacent interfering channels. The output of the filter 10 is supplied to an IF amplifier 11, which compensates for the insertion loss of the filter 10. The output of the amplifier 11 is supplied to a further surface acoustic wave filter 12, which provides further attenuation to the adjacent interfering channels. The output of the filter 12 is connected to a digital demodulator whose output is connected to the common display means 7. The demodulator 13 demodulates the received channel and contains an analog-digital converter (ADC). The demodulator may provide further gain.
In the arrangement shown in FIG. 1, two different tuners 4 and 9 are necessary because of differences in the performance requirements for digital and analog channels. For example, the digital tuner 9 generally requires a higher signal handling capability in order to cope with relatively high amplitude adjacent interfering analog signals. Also, the digital tuner 9 is generally required to have a superior local oscillator noise performance in order to meet the requirements of digital demodulation. Further, the digital tuner 9 may be required to cover only a portion of the band covered by the analog tuner 4.
The presence of the power splitter 2 inevitably degrades the signal quality to a greater or lesser extent. For example, the power splitter 2 is required to handle a relatively large input signal energy and intermodulation products generated within the splitter 2 degrade the received signals. Also, the power splitter 2 contributes its own thermal noise to the received spectrum and so degrades the noise performance of the tuner. Further, two independent tuners are required together with various surface acoustic wave filters and this adds to the size and cost of such an arrangement.
U.S. Pat. No. 6,147,713 discloses a multi standard television tuner capable of receiving analog or digital modulated signals. A tuner selects and converts a channel for reception to a conventional intermediate frequency, an example of which is given as 9 MHz. This is converted to digital in an analog-digital converter and the digital video signal is subjected to Nyquist lowpass filtering followed by decimation followed by further lowpass filtering.
U.S. Pat. No. 5,822,704 discloses a mobile telephone capable of receiving analog and digital standard signals. The telephone comprises a tuner which converts the selected channel to a conventional intermediate frequency and this is followed by quadrature detection. This detection process performs down-conversion to baseband. The resulting I and Q signals are converted to digital and further processed.